Adsorption characteristics of helium on an activated carbon at 4-10 K and its prospective application in 4 K-class regenerative cryocoolers

Adsorption data of helium on activated carbon at low temperatures are essential for its use in sorption coolers, gas-gap heat switches and the regenerators of cryocoolers. Especially when an activated carbon is used as the regenerator material in a cryocooler, the high specific heat caused by the high amount of adsorbed helium is expected to solve the bottleneck problem that the conventional 4 K cryocooler regenerator material suffers. It also has the advantage of being non-magnetic. However, the amounts of helium adsorbed on on the activated carbon in the temperature range of 4-10 K are low. In this paper, the adsorption isotherms of helium on a coconut shell-based activated carbon in the pressure-temperature range (4-10 K, 0.5-3.5 MPa) were measured with a self-built adsorption apparatus, from which the isosteric heats of adsorption and the specific heats of the activated carbon with adsorbed helium were calculated. The helium flow resistance through the activated carbon was also measured to evaluate its feasibility in a cryocooler. Results indicate that the isotherms are type I and can be fitted by the Langmuir adsorption model. The isosteric heats of adsorption are around 279 J/mol and change little with the amount of adsorbed helium. The specific heats of the activated carbon with adsorbed helium depend on the adsorption temperature and pressure, and their average values at 4-10 K are the highest among the commonly used solid regenerator materials (lead, SS, Er₃Ni and HoCu₂) and gaseous helium under the pressures investigated, which are ascribed to the high amount of helium adsorbed and the high heat of adsorption of helium on the activated carbon. The helium flow resistance through a material is related to its size and surface roughness and the value through the activated carbon is moderate among the four commonly used solid regenerator materials. Activated carbon with adsorbed helium is suitable as a regenerator material of cryocoolers because of its high specific heat and moderate flow resistance.